The present invention relates generally to radio communications systems and, in particular, to techniques which allow mobile stations to roam in areas having overlapping radio communications systems in which different types of communication systems and/or different frequency bands provide alternative radio communication coverage.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g., time division multiple access (TDMA) or code division multiple access (CDMA). Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analog standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardize design criteria for radio communication systems. Once created, these standards tend to be reused in the same or similar form, to specify additional systems. For example, in addition to the original GSM system, there also exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC in J-STD-007), both of which are based on GSM.
However, the most recent evolution in cellular communications services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States of America to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communications services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007), CDMA (IS-95) and D-AMPS (IS-136)), while others have been approved for the Cellular hyperband (e.g., AMPS (IS-54)).
Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of voice or speech channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control or voice channels may operate in either an analog mode, a digital mode, or a combination mode.
Historically, frequency bands in each cellular service area have been assigned to only one service company. For example, the A frequency band of the Cellular hyperband is usually reserved for use by non-wire line communications service companies, and the B frequency band is usually reserved for use by wire line communications service companies. Thus, if the service company providing cellular service to the subscriber is a wire line company, the Cellular hyperband mobile station is configured with the B frequency band as its "home" frequency band. Reciprocal billing arrangements between service companies allow subscribers to place calls over non-home frequency bands in the event the mobile station is roaming. These non-home calls, however, typically require payment by the subscriber of some form of a surcharge and are therefore undesirable. Furthermore, in the absence of an agreement between service companies, roaming subscribers may not be able to make a call without operator assistance. For the service provider, use of foreign frequency bands by subscribers results in a potential loss of revenue that the provider would like to avoid. Accordingly, cellular hyperband mobile stations have been configured to operate in a particular one of the available frequency bands within the Cellular hyperband.
The expansion to multiple hyperband communications capabilities as a result of the FCC's licensing of the PCS frequency bands has necessitated the development and placement into service of mobile stations that are capable of accessing both the Cellular and PCS hyperbands. To further complicate matters, different standards are being implemented in overlapping networks, e.g., an analog AMPS cellular base station on a first operator's network providing overlapping coverage with a PCS1900 base station connected to a second operator's network. Given this intermingling between standards and communication technologies, the number of different roaming permutations that must be addressed is significantly higher than those presented previously which, for example, only raised the issue of which frequency band to select. Thus, it would be desirable to provide techniques for determining which of a plurality of different types of networks a mobile station should connect with as that mobile station moves between various different types of overlapping service areas. Further, unlike existing roaming solutions, flexibility in re-prioritizing roaming options (both by the subscriber's operator and the subscriber) is desired.